diff --git a/shep/state.py b/shep/state.py
index a355c93..382ba07 100644
--- a/shep/state.py
+++ b/shep/state.py
@@ -1,5 +1,6 @@
 # standard imports
 import re
+import datetime
 
 # local imports
 from shep.error import (
@@ -39,6 +40,7 @@ class State:
         self.__keys = {getattr(self, self.base_state_name): []}
         self.__keys_reverse = {}
         self.__contents = {}
+        self.__change = {}
         self.verifier = verifier
 
 
@@ -302,6 +304,8 @@ class State:
         if contents != None:
             self.__contents[key] = contents
 
+        self.register_modify(key)
+
         return state
                                 
 
@@ -348,6 +352,8 @@ class State:
         current_state_list.pop(idx) 
         self.__add_state_list(to_state, key)
 
+        self.register_modify(key)
+
         return to_state
    
 
@@ -549,3 +555,11 @@ class State:
         """
         self.state(key)
         self.__contents[key] = contents
+
+
+    def modified(self, key):
+        return self.__change[key]
+
+
+    def register_modify(self, key):
+        self.__change[key] = datetime.datetime.now().timestamp()
diff --git a/shep/store/file.py b/shep/store/file.py
index f9b84d9..9dce8af 100644
--- a/shep/store/file.py
+++ b/shep/store/file.py
@@ -103,6 +103,16 @@ class SimpleFileStore:
         f.close()
 
 
+    def modified(self, k):
+        path = self.path(k)
+        st = os.stat(path)
+        return float(st.st_ctime())
+
+
+    def register_modify(self, k):
+        pass
+
+
 class SimpleFileStoreFactory:
     """Provide a method to instantiate SimpleFileStore instances that provide persistence for individual states.
 
diff --git a/tests/test_state.py b/tests/test_state.py
index f4a3acf..36e1e05 100644
--- a/tests/test_state.py
+++ b/tests/test_state.py
@@ -136,5 +136,21 @@ class TestState(unittest.TestCase):
         self.assertEqual(states.state('abcd'), states.FOO)
 
 
+    def test_change_dates(self):
+        states = State(3)
+        states.add('foo')
+        states.put('abcd')
+        states.put('bcde')
+
+        a = states.modified('abcd')
+        b = states.modified('bcde')
+        self.assertGreater(b, a)
+
+        states.set('abcd', states.FOO)
+        a = states.modified('abcd')
+        b = states.modified('bcde')
+        self.assertGreater(a, b)
+
+
 if __name__ == '__main__':
     unittest.main()